Electric motors operate through the interaction of opposing magnetic fields associated with a stationary element, a stator, most frequently generally resembling a hollow cylinder and a generally cylindrical rotating element, a rotor, whose axis lies on the axis of the stator. Either or both of these magnetic fields may be electrically generated by the passage of current through an electrical conductor.
More generally, multiple conductors are used, with the conductors configured to produce additive interaction of their individual magnetic fields.
These magnetic fields interact most effectively and generate maximum motor torque when shaped and focused by confining the cooperating conductors within a prescribed region carried on a low reluctance material such as an iron-based alloy. It will be appreciated that the conductors and their supporting low reluctance structure may be configured such that they constitute a rotor or a stator depending on the motor type.
A convenient way to shape the magnetic field of a stator or a rotor is to position the cooperating conductors in one of a plurality of slots radially distributed about the cylinder axis and extending along its length. The volume of the slot obviously limits the volume of electrical conductor which can be accommodated but other factors such as the form of the conductor are also important. For example, randomly-wound conductors with circular cross-section will not generally fill the slot cross-section as efficiently as placed or positioned conductors of square, rectangular or other parallel-sided cross-sections.
Yet further limitation results from the need to have the conductor loops insulated from one another and from the stator frame. Each conductor is coated with an electrically-insulating medium which beneficially electrically isolates it from its neighbors but increases conductor separation and hence limits the number of conductors which may be employed for any given slot dimension. There is therefore incentive to reduce insulation thickness as far as practicable.
Coating thicknesses may range from about 0.001 to 0.010 inches (about 0.025 to 0.25 millimeters) depending on the operating conditions of the motor and the dimensions of the conductor. These coatings are designed to afford good adherence to the conductor and to exhibit no cracks or exposed conductor at the conclusion of the motor manufacturing processes.
Nonetheless the combination of extensive handling and processing to which the conductor is subjected during motor manufacture, coupled with the thinness of the insulating coating can lead to rupture of or damage to the coating. Eventually such damage may lead to insulator breakdown, initiating short-circuits between adjacent conductors or between a conductor and the stator frame and resulting in the need for motor repair.
Thus, it would be advantageous if the dielectric properties of the insulation could be restored if the performance of the primary insulation is compromised
Further, in service, all motors are subject to ‘self-forces’ which arise from the interactions of the induced magnetic fields. These forces are applied to all current-carrying elements of the motor, including the windings. These forces promote relative movement of the individual windings and can lead to rubbing or fretting which eventually leads to insulation breakdown. Generally, provision is made to secure the windings and prevent their relative movement by tying the windings together or by encasing the windings in an at least somewhat rigid material such as epoxy or varnish or both in combination. However the magnitude of the forces, particularly in high current traction motors, all but assures that some conductor rubbing will occur.
Thus, for this reason also, it would be advantageous to automatically restore the dielectric properties of the insulation if the performance of the primary insulation is compromised.